1. Field of the Invention
The present invention relates to temperature control of machining liquid in a wire electric discharge machine.
2. Description of the Related Art
A wire electric discharge machine machines a workpiece to a desired shape by generating electric discharge by applying a voltage across a machining gap between a wire electrode and the workpiece while changing the relative position of the wire electrode with respect to the workpiece. Dimensional accuracy, perpendicularity, or angle accuracy is typically required of the machining result of the workpiece. To achieve the desired machining result, it is important to control the temperature of the machining liquid.
Temperature control of the machining liquid is important because the machining accuracy demanded of the wire electric discharge machine sometimes reaches the order of several microns in high accuracy machining, although it depends on the material, thickness, etc., of the workpiece. Any deformation of the workpiece due to variations in machining liquid temperature would degrade the machining accuracy with unacceptable dimensional error.
Variations in machining liquid temperature in the normally running wire electric discharge machine are mainly caused by the heat (temperature rise) produced during electric discharge machining. A technique for keeping constant the machining liquid temperature is known from Japanese Patent Application Laid-Open No. 2-124228 which relates to a machining liquid cooler. This technique suppresses the rise of the machining liquid temperature due to heat generated during machining. In contrast to the technique disclosed by Japanese Patent Application Laid-Open No. 2-124228, a technique for raising the machining liquid temperature is known from Japanese Patent Application Laid-Open No. 61-293723. The prior art techniques disclosed in Japanese Patent Applications Laid-Open Nos. 2-124228 and 61-293723 are techniques for controlling the machining liquid temperature in the wire electric discharge machine in the normal active state.
It is assumed here that, after being active, the wire electric discharge machine is inactivated for a predetermined time and then reactivated. In the inactive state, the machine is unattended and human operation of the machine (referred to hereinafter as “operation” for short) is not performed. In the inactive state, a machining liquid temperature regulator is stopped and the minimum electric power required to maintain the machine is consumed.
FIG. 5 shows an example in which the machine's ambient temperature is lower than a control target temperature of the machining liquid in the normal active state (referred to hereinafter as “machining liquid control temperature”). In this case, the machining liquid temperature in the inactive state lowers toward the ambient temperature and is controlled to reach the machining liquid control temperature after operation begins. As shown in FIG. 5, the states of the machine are classified into three:
stA: Normal active state. The machining liquid temperature is maintained at the machining liquid control temperature.
stB: Inactive state. Temperature control of machining liquid is stopped.
stC: Preparation-for-reactivation state. Restored from the inactive state, temperature control of machining liquid is resumed by the machining liquid circulating pump.
stD: Normal active state. The machining liquid temperature has reached the machining liquid control temperature and the normal active state is restored (i.e., stA and stD are identical states).
The labels in FIG. 5 have the following meanings:
T1: Machining liquid temperature when the machine is reactivated,
T2: Machining liquid control temperature,
t1: Time at which the machine is inactivated,
t2: Time at which preparation to reactivate the machine starts, and
t3: Time at which the machining liquid temperature reaches the machining liquid control temperature.
If the machining of workpiece is started in the preparation-for-reactivation state stC before the machining liquid temperature reaches the machining liquid control temperature T2, the machining liquid temperature at the start of machining is different from the temperature at the end of machining. Temperature variations of the machining liquid deform the workpiece, resulting in a machined product with a significant dimensional error. To avoid any machining result with unacceptable dimensional error due to premature machining started in the preparation-for-reactivation state (stC), it is arranged that machining does not start before the machining liquid temperature reaches the machining liquid control temperature.
To raise the machining liquid temperature from a machining liquid temperature lower than the machining liquid control temperature as shown in FIG. 5, there is a method using a machining liquid circulating pump, for example. However, this method does not efficiently raise the machining liquid temperature, because it uses the heat generated by the operation of the machining liquid circulating pump, i.e., by the secondary effect of the operation of the machining liquid circulating pump. To aim only at raising the machining liquid temperature, a warming device such as a heater as disclosed by Japanese Patent Application Laid-Open No. 61-293723 can raise the machining liquid temperature to the control temperature in a shorter time.
FIG. 6 is a schematic diagram showing that a warming device can shorten the preparation-for-reactivation state (stC) from t3-t2 to t4-t2.
The labels used in FIG. 6 have the following meanings:
stC2: Preparation-for-reactivation state (with warming device), and
t4: Time at which the machining liquid control temperature is reached (with warming device).
In the preparation-for-reactivation state above, the machine has been restored from the inactive state and machining liquid temperature control is resumed by the warming device and the machining liquid circulating pump.
The description for FIG. 5 applies to the other labels in FIG. 6 common to those in FIG. 5.
Even if the warming device is employed, the machining liquid temperature begins to be raised by the preparation-for-reactivation operation. A certain preparation-for-reactivation time period exists, therefore, after the operation begins until the machining liquid control temperature is reached, during which machining cannot produce a machining result with high accuracy.
To solve such a problem, a technique for controlling the machining liquid temperature using a timer function prior to the beginning of operation is known from Japanese Patent Application Laid-Open No. 61-297033. FIG. 7 is a schematic diagram showing the technique for controlling the machining liquid temperature using the timer function disclosed by Japanese Patent Application Laid-Open No. 61-297033, in comparison with the case in which no timer function is used.
The labels used in FIG. 7 have the following meanings:
stC2: Preparation-for-reactivation state (without timer function),
stC3: Preparation-for-reactivation state (with timer function),
t5a: Preparation-for-reactivation start time (with timer function),
t6: Time at which the control temperature is reached (with timer function),
t2: Preparation-for-reactivation start time (without timer function), and
t4: Time at which the control temperature is reached (without timer function).
The preparation-for-reactivation start time (without timer function) above is the time at which human operation begins.
The description for FIG. 5 applies to the other labels in FIG. 7 common to those in FIG. 5.
The temperature control of machining liquid shown in FIG. 7 will now be described. It is assumed that operation begins at time t2. When temperature control of machining liquid is resumed at time t5a in a machine having the timer function, the machining liquid control temperature T2 is reached at time t6. Since time t6 is earlier than the operation beginning time, machining can be started immediately after the operation begins. On the other hand, when the machining liquid temperature begins to be raised at time t2 in a machine having no timer function, machining cannot be started until the machining liquid control temperature T2 is reached at time t4.
In this way, in the machine having the timer function, machining can be started without delay after the operation begins. This technique requires, however, for the operator to set the time for the timer function. This involves a problem that it is difficult to precisely determine how much the machining liquid temperature has changed during the inactive period, how much the current temperature deviates from the target temperature, and how much time is required to reach the target temperature.
FIG. 8 illustrates a prior art technique for controlling the machining liquid temperature by varying the timing for enabling the timer function. It is assumed here that operation begins at time t2. The timer function is enabled at time t5a, t5b, or t5c. When preparation-for-reactivation starts at time t5a, the control temperature is reached before the operation begins and electric power is wasted by the operation of the machining liquid circulating pump until time t2 (i.e., during period D2). When preparation-for-reactivation starts at time t5b, the machining liquid control temperature is reached at time t2. Since the operation begins at time t2, the warming device and machining liquid circulating pump do not uselessly operate. When preparation-for-reactivation starts at time t5c, the machining liquid control temperature cannot be reached by the time the operation begins, so machining cannot be started until the machining liquid control temperature is reached at time t7 (i.e., during period D3). Since in the technique disclosed by Japanese Patent Application Laid-Open No. 61-297033, the timer may be set to any point in time, it is not clear to which of time t5a, t5b, or t5c in FIG. 8 the timer is set. It is impossible to set the timer precisely by predicting in advance how much the machining liquid temperature changes during the inactive period.
A technique for solving this problem is disclosed by Japanese Patent Application Laid-Open No. 2010-105101. This technique relies on a temperature measuring means for measuring the temperature at or near a machine component or part. If the machining liquid is circulated for pre-conditioning before the program starts, the time required for pre-conditioning circulation is predicted using the difference between the temperature measured by the measuring means and the preset machining liquid temperature and then the pre-conditioning circulation is performed for the predicted time period until the program starts.
The technique disclosed by Japanese Patent Application Laid-Open No. 2010-105101, however, does not take into consideration the amount of machining liquid. The time required to change the machining liquid temperature varies with the amount of machining liquid. At a site where the wire electric discharge machine is actually used, even if the machining liquid is stored to the maximum capacity when the machine is installed, the amount of machining liquid may change due to evaporation etc., so it is quite unlikely that the amount of machining liquid is the same in all wire electric discharge machines in the world. The timer function cannot be operated at an appropriate time, therefore, using only the table in the technique disclosed by Japanese Patent Application Laid-Open No. 2010-105101.
Furthermore, according to the technique disclosed by Japanese Patent Application Laid-Open No. 2010-105101, temperature control of the machining liquid remains the same after the control is started. Since there is no information of the amount of machining liquid as described above, the control target temperature could be reached earlier than predicted. When the control target temperature is reached earlier than predicted, electric power would be wasted for machining liquid circulation etc. until the operation begins.
Although the above description is about the case in which the machining liquid temperature in the inactive state is lower than the control temperature, a similar description applies to the case in which the machining liquid temperature in the inactive state is higher than the control temperature.